Dry deposition to an urban complex

The importance of dry deposition in consequence assessment is discussed, with particular attention given to the distinguishing features of the urban case. The applicability in this situation of the concept of dry deposition velocity is assessed and found to require careful interpretation. Then, field measurements - both techniques and results - are discussed, highlighting the difficulties of making generalisations from the current sparse data base. The utility of wind-tunnel modelling is examined, and the problems of ensuring adequate similarity between full and reduced scale are pointed out. The currently favoured technique for deducing an urban deposition velocity -which equates it to the area-weighted aggregate of the deposition velocities for individual surface types in the city - is investigated; it is shown not to be a rigorously defensible method, although it may yield a sufficiently good estimate for certain applications. Finally, mathematical models for deposition to arrays of bluff objects are discussed. It is shown that the techniques for estimating deposition to vegetative canopies, as proposed to data, do not appear to have much to offer in the case of bluff elements. Models specifically developed for bluff elements are found not to be sufficiently detailed nor well-developed yet to make them predictive tools. (author)